1. Field of the Invention
This invention relates generally to the field of digital signal processing and more particularly to a concatenated equalizer/trellis decoder suitable for decoding multiple mode trellis encoded High Definition Television (HDTV) signals.
2. Background of the Invention
The Advanced Television Systems Committee (ATSC) standard for HDTV in the United States specifies an eight (eight levels per symbol) vestigial sideband (VSB) transmission system as described in the “ATSC Digital Television Standard”, Document A/53 published on Sep. 16, 1995. This document sets forth all the requirements regarding HDTV signal characteristics. In the receiver, an equalizer is included which is an adaptive filter which receives the VSB data stream at an average rate equal to the symbol rate of approximately 10.76 MHz. The equalizer attempts to remove linear distortions mainly caused by multipath signal propagation, which is characteristic of terrestrial broadcast channels. One equalizer design suitable for use in an HDTV receiver is a decision feedback equalizer (DFE) as described in John G. Proakis, “Digital Communications”, McGraw-Hill, 2nd edition, 1989, New York and in U.S. Pat. no. 6,493,409, entitled PHASE DETECTORS IN CARRIER RECOVERY FOR OFFSET QAM AND VSB, issued on Dec. 10, 2002 to Lin et al. A simplified block diagram of a typical DFE architecture is shown in FIG. 1. The DFE is seen to include a Feed Forward Filter (FFF), a Feedback Filter (FBF), a slicer, a lock detector and a mode switch, and is capable of operating in training, blind or decision directed (dd) modes.
The functions of the FFF, FBF and the slicer are well known and together they perform the basic functions of filtering and quantization. The lock detector compares the equalizer output and the slicer levels with a threshold, and in response to that operation generates an updated lock detector output. The mode switch chooses the appropriate input to the FBF filter as well as selecting the error and control signals to be used in performing the equalizer adaptation according to the current equalizer operating mode. The mode switch also examines the lock detector output. In normal operation, the equalizer mode switch has an automatic switching capability, which depends on the status of the equalizer lock detector. The mode switch assumes that the training and blind modes are used for convergence purposes only. After the equalizer lock detector senses convergence, the equalizer is then switched to the decision directed (dd) mode. Whenever convergence is lost, the mode switch returns the equalizer to the training or blind mode.
In the ATSC standard, a training sequence is included in the field sync signal in order to provide a mechanism for initial equalizer convergence. The received sequence of coded symbols serves as the input to a synchronization control unit, which detects field and segment synchronization patterns within the symbol sequence and generates the corresponding sync signals. In the training mode, the equalizer coefficients are only updated during the field sync pulse. The drawbacks of this method are that it requires the prior correct detection of the field sync, and since the training sequence is contained in the field sync, which occurs only once every 25 milliseconds, the rate of convergence is possibly decreased. In an environment of multiple reflected or ghost signals, or in other dynamic environments, the detection of the field sync may be difficult. In those situations the receiver needs some self recovering or blind method of initially adjusting equalizer tap coefficients without referring to the training sequence. Since a blind algorithm works with every data symbol it will also tend to have a faster rate of convergence. An example of a blind convergence algorithm is Godard's Constant Modulus Algorithm (CMA). See D. N. Godard, “Self-Recovering Equalization and Carrier Tracking in Two Dimensional Data Communication Systems”, IEEE Transactions on Communications, Vol. COM-28, pp.1867-1875, November 1980. See also D. N. Godard, U.S. Pat. No. 4,309,770.
The final mode of equalizer operation, decision directed (dd), assumes that the input to the feedback filter (FBF) is the output of the slicer. Since the adaptation error and the input to the feedback filter are aided by the presence of the slicer, coefficient adaptation occurs throughout the data sequence. The dd mode does not have good convergence characteristics, but once convergence is achieved it has advantages when compared to the other modes of equalizer operation. The presence of the slicer data results in a reduced mean squared error (MSE) and bit error rate (BER) at the equalizer output when compared to operation in the blind mode. Since the dd mode updates its coefficients with every symbol rather than interpreting just the training symbols, the dd mode provides faster adaptation and tracking capabilities than the training mode.
Trellis coding is used in combination with other techniques to protect against interference from particular noise sources. Trellis coding requirements for HDTV are presented in sections 4.2.4-4.2.6 (Annex D), 10.2.3.9, 10.2.3.10 and other sections of the Digital Television Standards for HDTV Transmission of Apr. 12, 1995 prepared by the ATSC. The HDTV standard presents a trellis coding system that employs an interleaving function involving twelve parallel trellis encoders at a transmitter and twelve parallel trellis decoders at a receiver for processing twelve interleaved data streams. The trellis system employed utilizes a rate ⅔ trellis coded modulation (TCM) code. The code is implemented by coding one bit using a rate ½, four state convolutional encoder, and then adding an FEC uncoded bit which is differentially precoded. Each set of three coded bits produced by the encoder is mapped to an eight level VSB modulator symbol. FIG. 2 is a block diagram showing the differential precoder, trellis encoder and corresponding eight level VSB symbol mapper. The twelve identical encoders and precoders are used sequentially, processing each one byte at a time and subsequently transmitting one complete symbol at a time. For each encoder, the input data bits X1 and X2 are encoded as three bits Z2, Z1, and Z0. Each three-bit word corresponds to one of the eight symbols R. The input bit X2 is processed by a precoder to provide encoded bit Z2. The input bit X1 is encoded as two bits Z1 and Z0 by the trellis encoder. An example of a trellis decoder used in an HDTV receiver is disclosed in U.S. Pat. No. 5,841,478, entitled CODE SEQUENCE DETECTION IN A TRELLIS DECODER, issued on Nov. 24, 1998 to Hu, et al.
The use of DFE techniques has been the subject of controversy in the field of receiver design. While DFE offers a relatively simple method for equalizing a highly dispersive linear channel, it may suffer from error propagation, a mechanism whereby incorrect source symbol estimates may cause future decision errors leading to potentially lengthy error bursts. In the HDTV receiver, when the terrestrial channel introduces multipath and white noise, particularly when the multipath signal is strong and the signal to noise ratio (SNR) is low, error propagation in the feedback filter (FBF) of the equalizer (DFE) affects the performance at the output of the trellis decoder. Simulation of an ATSC receiver including a Decision Feedback Equalizer (DFE) for an HDTV terrestrial channel having strong multipath and Additive White Gaussian Noise (AWGN) shows that receiver performance can be improved if the decision directed mode is replaced by a soft decision directed mode, whereby the input to the FBF filter is the equalizer output instead of the slicer output. In addition, ideally the equalizer feedback filter should receive more accurate symbol decisions than those provided by the equalizer slicer.